Fluorescence refers to the short-duration, spontaneous emission of light of one wavelength upon excitation by another, shorter wavelength of light. Substances that fluoresce are generally referred to as fluorophores. There are numerous natural and synthetic fluorophores, including, but not limited to, chemical dyes and mineral substances.
Conventional fluorescence standards or fluorescent dyes generally are composed of organic compounds with extended aromatic, pi bonding structures. The basis for fluorescence in these dyes is that the molecules absorb light of a given wavelength range (e.g., UV or visible light) and re-emit a portion of the absorbed energy at a known, different wavelength, often a longer wavelength. Absorption of light generally leads to excitation of electrons in the pi structure to a higher energy state; re-emission occurs when the electrons relax to their ground state. These dyes are used in a variety of different biological assays, for example, where the fluorescence signals they emit can provide information about the system under study.
A number of mineral substances are also known to fluoresce. However, most minerals do not fluoresce when pure. It takes certain impurities in certain quantities to make the mineral fluoresce. Such impurities are called “activators.” Different activators can make the same mineral fluoresce in different colors. There are a few minerals that will fluoresce when pure. These are called “self-activated” minerals, and include scheelite, powellite, and several uranium minerals. Other common fluorescent minerals include calcites (with a variety of activators), rubies, and sapphires.
However, many chemical fluorescence standards and fluorescent dyes have one or more of the following disadvantages: (A) They are good at time zero, but they are not stable over the long term. That is, the dyes decay rapidly, they are subject to photobleaching (especially under extended illumination and when illuminated with high intensities), (B) they are usable only within a narrow spectral range (additional excitation/emission combinations generally require additional dyes), (C) they are costly, and (D) they are mechanically, thermally, or chemically unstable, and can age or dry out, which results in a change in the fluorescence intensity. Mineral standards (e.g., uranium glass) are generally considered to be preferable to dyes in that they are chemically stable. Nevertheless, some mineral standards may suffer from poor uniformity in their bulk material and they can be fragile (e.g., uranium glass is easily broken).